1. Field of the Invention
The present invention relates to an optical system having the function of automatically compensating for the shaking of a photographed image caused by the vibration of the optical system, i.e., the so-called image stabilizing function, and is more particularly directed to, for example, a 4-unit zoom lens whose back focal length is shortened to a compact size and which has a zoom ratio of about 5, wherein one of the lens units is made to move in directions perpendicular to an optical axis, while preventing the deterioration of optical performance from occurring when the image is stabilized, suitable for use as the optical system in silver-halide photography cameras, video cameras, electronic still cameras and the like.
2. Description of Related Art
When the housing of a photographic system tilts due to vibrations, the image at the focal plane changes its position by a distance depending on the tilt angle and the focal length of the photographic system. With a photographic apparatus for taking still pictures, therefore, in order to avoid deterioration of image quality, there has been a problem that the exposure time must be made short enough. Meanwhile, the motion picture camera has the problem that vibrations make it difficult to maintain the setting of a desired composition. For these reasons, a necessity arises for automatically compensating for the photographic system so that, even when the photographic system tilts due to vibrations, a change of the position of an image at the focal plane, i.e., the "shaking" of a photographed image, does not occur.
Attempts have been made to compensate for the image shaking by driving an optical element constituting part of the photographic system. As the arrangement of the optical element differs with different optical systems, a wide variety of methods of optically compensating for image shaking have been proposed.
In general, such optical methods of image stabilization have important prerequisites that the operating mechanism be relatively simple in construction, that the optical element to be used be as light in weight as possible, that the amount of driving be as small as possible, and that good optical performance be maintained against vibrations throughout as wide a range of tilt angles as possible. Also, in recent years, the development of high-range zoom lenses is flourishing. With such lenses, particularly in a telephoto setting, a large amount of image shaking is likely to occur. Because of this, a capability of compensating for image shaking is taken seriously. So, other attempts have been made to reconcile the above-described prerequisites and the extension of the zooming range toward longer focal lengths. With this in mind, many zoom lenses have been proposed.
Here, as the optical image stabilizing methods, there are a shift-image stabilizing method in which one of the lens units as a whole or in part is displaced in directions perpendicular to the optical axis, a tilt image stabilizing method in which one of the lens units as a whole or in part is rotated about an axis perpendicular to the optical axis, a variable-angle-prism image stabilizing method in which a variable-angle prism formed by filling a liquid of relatively high transmittance, such as silicon oil, in between a pair of transparent plates, is disposed near or inside the photographic optical system and the transparent plate is made to tilt with regard to the optical axis so as to effect image stabilization by the prism action, and so on. Many examples of application of these methods to zoom lenses are also known.
Among such methods of optically compensating for image shaking, the shift-image stabilizing method is widely used, although it is slightly inferior in the degree of freedom of aberration correction to the tilt image stabilizing method that allows relatively free choice of a position at which to put the axis of rotation, because the shift-image stabilizing method has the advantage that the operating mechanism for the lens unit is relatively simple. Another advantage of the shift-image stabilizing method is that, as compared even with the variable-angle-prism image stabilizing method, the chromatic aberrations of the stabilized optical system can be corrected with a greater degree of freedom. Thus, the shift-image stabilizing method has a relatively simple form, and still gets a relatively high optical performance.
Image-stabilizing zoom lenses employing such a shift-image stabilizing method have been proposed in, for example, Japanese Laid-Open Patent Applications No. Hei 6-265827, No. Hei 7-318865 and No. Hei 8-82769.
In Japanese Laid-Open Patent Application No. Hei 6-265827, there is disclosed a zoom lens comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit of positive refractive power and a third lens unit of negative refractive power, totaling three lens units. During zooming from the wide-angle end to the telephoto end, the separation between the first and second lens units increases, while the separation between the second and third lens units decreases. The second lens unit is divided into a front lens subunit and a rear lens subunit. The rear lens subunit is made to shift so as to effect image stabilization.
In Japanese Laid-Open Patent Application No. Hei 7-318865, there is disclosed a zoom lens comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, a fourth lens unit of positive refractive power and a fifth lens unit of negative refractive power. During zooming from the wide-angle end to the telephoto end, at least the first and fifth lens units are made to move toward the object side in such relation that the separation between the first and second lens units increases, the separation between the second and third lens units decreases, the separation between the third and fourth lens units increases and the separation between the fourth and fifth lens units decreases. The fourth lens unit is made to shift so as to effect image stabilization.
In Japanese Laid-Open Patent Application No. Hei 8-82769, there is disclosed a zoom lens comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit of positive refractive power, a stop and a third lens unit of negative refractive power. During zooming from the wide-angle end to the telephoto end, the separation between the first and second lens units increases, while the separation between the second and third lens units decreases. The second lens unit is made to shift so as to effect image stabilization.
The zoom lens proposed in Japanese Laid-Open Patent Application No. Hei 6-265827 has a zoom ratio as low as 2.6. Assuming that, with this configuration left unchanged, the zoom ratio is increased to 5 or thereabout, then the aberrations occurring during image stabilization, of course, and the ones in the normal state, too, deteriorate greatly. Hence, there is a problem that the aberrations can not be corrected well.
Also, because the image-stabilizing lens unit has a large number of constituent lenses, the lens weight is relatively heavy. Then, supposing the zoom ratio is increased and all aberrations occurring during image stabilization are corrected, a problem arises in that the number of constituent lenses increases greatly, which, in turn, causes an unduly great increase of the load on the operating mechanism during image stabilization.
Further, the zoom lens proposed in Japanese Laid-Open Patent Application No. Hei 7-318865 has a slightly higher zoom ratio at about 3.3. The form of the image-stabilizing lens unit is relatively simple. However, the aberrations deteriorate greatly during image stabilization. Therefore, a problem arises in that a further increase of the zoom ratio makes the deterioration of the aberrations unacceptable.
Further, the zoom lens proposed in Japanese Laid-Open Patent Application No. Hei 8-82769 is relatively high in the zoom ratio; particularly, for the embodiment 2 thereof, the zoom ratio is about 3.9. The aberrations occurring during image stabilization are corrected relatively well over a relatively wide range of angles of image stabilization. However, a large number of constituent lenses are used in the image-stabilizing lens unit. Therefore, a problem arises in that a large load is put on the operating mechanism during image stabilization. Moreover, in order to increase the zoom ratio to 5 or thereabout, the number of constituent lenses increases greatly, causing a great increase of the size of the lens system. It is, therefore, difficult to correct the aberrations for both of the normal state and the image stabilizing state while maintaining simple and compact form.